Introduction

Factors affecting Boiler Performance

Testing Techniques & Performance Optimisation

Boiler Performance

Presentation Coverage

200 MW BHEL Boiler

500 MW BHEL Boiler

500 MW Talcher NTPC

Factors affecting Boiler Performance

IntroductionBoiler performance depends on

Boiler design

Coal Quality Operating practices / parameters

Component condition

Boiler Performance Characterisation

Combustion / Thermal Efficiency - Conversion of chemical heat in fuel to production of steam adequate Time / Temperature / Turbulence

Auxiliary Power Consumption The total power being consumed by ID, FD, PA fans and the mills.

OFF Design/Optimum Conditions

Parameter Deviation Effect on Heat Rate

Excess Air (O2) per % 7.4 Kcal/kWh Exit Gas Temp per oC 1.2 Kcal/kWh Unburnt Carbon per % 10-15 Kcal/kWh Coal moisture per % 2-3 Kcal/kWh Boiler Efficiency per % 25 Kcal/kWh

Effect of Boiler side Parameters (Approx.)

Boiler Control Volume

Factors affecting Boiler efficiency include

Design Coal Quality Mill Performance - PF Fineness Burner-to-burner PF balance Excess Air Level Boiler Air Ingress AH Performance Furnace / Convective section Cleanliness Quality of Overhauls Water Chemistry, boiler loading, insulation etc.

Efficiency Vs Moisture in Coal

AssumptionsExit Gas Temp - Constt.Fuel Moisture - 20.5 %Excess Air - 20 %GCV - 3700 kal/kg

Efficiency Vs Hydrogen in Coal

AssumptionsExit Gas Temp - Constt.Fuel Hydrogen - 2.33 %Excess Air - 20 %GCV - 3700 kal/kg

Efficiency Vs HHV of Coal

AssumptionsExit Gas Temp - Constt.Fuel Moisture - ConsttFuel Hydrogen - ConsttExcess Air - 20 %GCV - 3700 kal/kg

Efficiency Vs Excess Air

AssumptionsExit Gas Temp - Constt.Ambient Temp - 27 CGCV - 3700 kal/kg

Efficiency Vs Ambient Temp / RH

AssumptionsExit Gas Temp - Constt.Excess Air - 20 %GCV - 3700 kal/kg

Proximate Analysis, Ultimate Analysis, Calorific Value, Ash

Constituents, Ash Fusion Temperatures, FC/VM ratio, Hard Grove

Index, YGP (Yeer Geer Price) Index

Typical Proximate Coal Analysis - Fixed Carbon - 32.4 %, Volatile

matter - 21.6 %, Moisture 16.0 %, Ash 30.0 %, GCV 4050 kcal/kg

+ve aspects - Low Sulfur, Low chlorine, Low iron content and High Ash

fusion temp

-ve aspects - High ash, moisture, high silica / alumina ratio, low calorific

value, high electrical resistivity of ash,

Problem

Variation in heating values, moisture, ash content and volatile matter

The Coal

CAsh H O N S Mi M

As fired basisAir dry basisDry basis

Dry & Ash free basis

A FC VM MCoke Volatile

Ultimate

Proximate

Coal Composition -Different bases of representation

Changes in Coal Quality - Coal characteristics decide the heat release rates, furnace wall conditions and consequently the

furnace heat transfer

Deterioration in Coal quality affects boiler capability to operate at

rated parameters.

Change in coal quality affects capacity, efficiency and combustion stability.

Increase in moisture affects mill drying, tempering air requirement, gas velocities, ESP & Boiler efficiency.

Ash quality / quantity affects boiler erosion, mill wear, slagging and fouling propensity, ash handling system, sprays, sootblowing etc.

Change in coal characteristics affects mill wear parts life & throughput of Pulverizers.

Increased dust loading & change in dust characteristics may affect ESP performance.

FACTORS AFFECTING MILL PERFORMANCE

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FINENESS - % THRU 200 MESH

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HARDGROOVE INDEX (HGI)

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GRINDABILITY (HGI)

FINENESS

MOISTURE

SIZE OF RAW COAL

MILL WEAR (YGP)

MTC PRACTICES

PF fineness

Fineness is expressed as the percentage pass through a 200-mesh screen (74m).

Coarseness is expressed as the percentage retained on a 50-mesh screen (297m).

Screen mesh - number of openings per linear inch.

Typical recommended value of pulverised fuel fineness through 200 mesh Sieve is 70% and 1% retention on 50 mesh sieve.

Flyash is over 80% of total ash, So its important to test for unburntcarbon; For monitoring unburnts in bottom ash, a visual in shift beginning or after mill change overs is required.

PF fineness is influenced by

Coal Quality

Mill loading, settings, mill problems

PA flows / velocities

Sampling Techniques

Conventional Cyclone / ASME Sampler

64 point rotary sampler

Sampling location

Near mill / burner

single pipe / average

Manual / motorised sieve shaker

EFFECT OF FINENESS ON BOILER OPERATION

Excessive PF fineness would cause

Reduction in mill capacity Increased mill component wear Increased mill and fan power combustion

Excessive PF fineness may not necessarily result in improved combustion

Control Room

Boiler

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2 3

4

Mills

Mill discharge pipes offer different resistance to the flows dueto unequal lengths and different geometry layouts.

Fixed orifices are put in shorter pipes to balance velocities / dirty air flow / coal flows. The sizes of the orifices are specified by equipment supplier.

A B C D E F

Burner Imbalance

TANGENTIAL FIRINGUneven fuel and air distribution can result in

High unburnt carbon in flyash

Non - uniform release and absorption of heat across the furnace resulting in temperature imbalance

Reducing furnace leading to slagging and fouling

High furnace and boiler exit gas temperatures

Water wall wastage and tube metal overheating

Burner Imbalance

Primary Air Flow

Coal Flow

Dirty air flow distribution should be with in +/- 5.0%of the average of fuel pipes

Coal distribution should be with in +/-10% of the average of fuel pipes

Balanced Clean air flows do not necessarily result in balanced Dirty air flows.

Instruments for on line measurement of PF flow based Electrostatic detection, Microwave injection and Acoustics are commercially

available.

Rotary Sampler (For coal sample from mill discharge pipes)

Demo in Mill 1A Unchahar (May07-Oct07) Real Time feedback on Dirty air & PA flow velocities in PF pipes Dirty Air balance by use of variable orifices in PF pipes Accurate Primary Air Flow Measurement (Electrostatic detection)

System Hardware 2X2 sensors in PA duct 2X1 sensors in each discharge pipe of

Mill 1A Instrument Cabinet in Control room Manually operated orifices in PF pipes

Burner PF balance checking Tests 210MW (May06) Measured temperatures of dirty air at burners were lower than mill

outlet temperatures; Attended during unit s/d

Mill 5 A Comparison of Mill outlet temp with individual pipe temp

Test Date UCB Temp C

Corner 1

Corner 2

Corner 3

Corner 4

Diff.C

24.02.06 90 72.1 76.6 74.1 71.3 16

18.05.06 86 83.6 83.2 82.1 81.6 3.3

High absolute velocities of dirty air ~ 28-30 m/s (High PA header pressure ~ 930 mmWC)

+50 fineness fractions of mills D & E ~ 5% - high unburnt C in bottom ash

Excess Air

Typically 20 % excess air is recommended for boiler operation; Actual optimal value would vary from boiler to boiler depending on coal quality, fineness and other operating practices.

Optimum level of oxygen could be less than value specified by OEM.

O2 instruments are installed at the economizer exit, where they can be influenced by air infiltration. The O2 reading in control room may not be necessarily representative of the actual O2 in furnace.

Variation of Oxygen & Temp across at RH Inlet Left & Right side 210 MW (May'07)

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UCB O2 (L/R): 1.8/2.1 %After Zir. calibration: 3.4/3.35 %

Excess air is amongst the most important factors affecting boiler performance

Excess Air - CO monitors a must for boilers

C + O2 = CO2 + 8084 kcal / kg of Carbon2C+ O2 = 2CO + 2430 kcal / kg of Carbon2H2+ O2 = 2H2O + 28922 kcal / kg of HS + O2 = SO2 + 2224 kcal / kg of Sulphur

All boilers need to be equipped with On line CO monitors at Eco Outlet / ID fan discharge. We lose 5654 kcal for each kg of CO formed.

Ideally, average CO at gooseneck after combustion completion should be below 100 ppm and no single value over 200 ppm

Excess Air

Low excess air operation can lead to unstable combustion (furnace puffs) slagging of waterwalls and SH sections loss in boiler efficiency due to increased CO / unburnt

combustibles

High excess air operation can lead to Increased boiler losses High SH / RH temperatures Higher component erosion

Boiler Air Ingress Cold air leaks into the boiler from openings in the furnace and

convective pass and through open observation doors. Some of the boiler leakage air aids the combustion process;

some air that leaks into the boiler in the low temperature zonescauses only a dilution of the flue gas.

This portion of air appears as a difference in O2 level between the furnace exit and oxygen analysers at economizer exit. Actual oxygen in the furnace could be much less.

Also, boiler casing and ducting air ingress affects ID fans power consumption and margins in a major way.

Air-in-leakage

Furnace Outlet

Zirconia O2 Probe

AH Seal Lkg

ESP

Expansion Joints

Air Ingress Points Furnace Roof , Expansion joints, Air heaters, Ducts, ESP Hoppers, Peep Holes, Manholes, Furnace Bottom

Typical Air ingressPenthouse & 2nd pass ~ 0-5%Air heaters ~ 12-20% (tri sector)AH outlet to ID suction ~ 5 to 9%.

The difference between oxygen at furnace outlet (HVT) and economizer outlet (zirconia) was in the range of 1.0 to 2.5 % in many boilers.

Apart from degradation of AH baskets performance, another reason for lower heat recovery across air heaters is boiler operation at lesser SA flows due to high air-in-leakage.

Replacement of Metallic / Fabric Expansion joints in 10 years / 5 years cycle recommended.

Air Ingress

Air ingress can be quantified by the increase in oxygen % in flue gas; The temperature drop of the flue gas from air heater outlet to ID fan discharge also provides an indication of the same.

Oxygen % at various locations in boiler

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Furn Outlet AH Inlet AH Outlet ID outlet

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210 MW 210 MW 500 MW 210 MW

Boiler Air Ingress

Air HeatersFactors affecting performance include Operating excess air levels PA/SA ratio Inlet air / gas temperature Coal moisture Air ingress levels Sootblowing No. of mills in service

Air HeatersFactors affecting performance include

PA Header PressureHigh pressure results in increased AH leakage, higher ID fan loading, higher PA fan power consumption, deteriorates PF fineness & can increase mechanical erosion

Upstream ash evacuation

Maintenance practicesCondition of heating elements, seals / seal setting, sector plates / axial seal plates, diaphragm plates, casing / enclosure, insulation

Boiler Exit Gas Temperature

Ideal flue gas temperature at stack outlet should be just above the dew point to avoid corrosion; Higher gas temperatures reduce efficiency; Possible causes of temperature deviations are

Dirty heat transfer surfaces High Excess air Excessive casing air ingress Fouled/corroded/eroded Air heater baskets Non - representative measurement

Contd..

AUXILIARY POWER CONSUMPTION

Major auxiliaries Consuming Power in a Boiler are FD fans, PA fans, ID fans and mills. Reasons for higher APC include

* Boiler air ingress* Air heater air-in-leakage* High PA fan outlet pressure* Degree of Pulverisation* Operation at higher than optimum excess air

Main Steam/ Reheated Steam Temperature

While an increase in steam temperatures is beneficial to Turbine Cycle Heat Rate, theres no benefit to boiler efficiency, infact it affects reliability adversely.

Testing Techniques & Performance Optimisation

Test Objective To generate feedback for opn & mtc.

To determine current boiler efficiency levels To determine each component of the heat loss to find

the reasons for deterioration To establish the cost / benefit of annual boiler O/H To establish baseline performance data on the

boiler after major equipment modifications To build a database for problem solving and

diagnosis

Suggested Frequency of Testing

QuarterlyBoiler Efficiency

Pre/Post O/H & Six monthly

FG Path O2mapping

QuarterlyAH Perf. Test

Pre/Post O/HDirty Air FlowFrequency

Boiler & Air Heater TestsTests to be conducted under defined operating regime (O2level / PA Header Pressure / no. of mills) at nominal load

Pre Test Stabilisation Period

Prior to the test run, equipment must be operated at steady state conditions to ensure that there is no net change in energy stored in steam generator envelope.

Minimum Stabilisation Time - 1 hour

Pre Test Checks

Sootblowing completed at least one hour before start of the test Steam coil air preheaters steam supply kept isolated All feedwater heaters in service with normal levels, vent settings

and with normal drain cascading No sootblowing or mill change over during the test. In case oil guns

are used, the test shall be repeated Air heater gas outlet dampers are modulated to ensure minimum

opening of cold air dampers to mills Auxiliary steam flow control kept isolated or defined during the test. CBD / IBD blowdowns kept isolated for the test duration Bottom hopper deashing after completion of test and not during the

tests

Test Duration

Should be sufficient to take care of deviations in parameters due to controls, fuel variations & other operating conditions.

When point by point traverse of Flue gas ducts is done, test should be long enough for atleast two traverses.

In case of continuous Data Acquisition System & use of composite sampling grids, shall be based on collection of representative coal & ash samples.

Could be 1/2 to 2 hours in case of parametric optimisation tests or 4 hours for Acceptance Tests.

Frequency of Observations

Parameter readings to be taken at a maximum interval of 15 minutes & a preferred interval of 2 minutes or less

Measurements during a Boiler Test Coal Sample for Proximate analysis & GCV Bottom Ash and Flyash Samples Flue Gas Composition at AH Outlet Flue Gas Temperature at AH Inlet / Outlet Primary / Secondary air temp at AH inlet / outlet Dry / Wet bulb temperatures Control Room Parameters

(All measurements / sampling to be done simultaneously)

Coal Sampling

Coal Samples are drawn from all individual running feeders from sampling ports in feeder inlet chutes

Composite sample is collected from all running feeders

One sample is sealed in an air tight container for total moisture determination

Flyash Sampling

Flyash is collected in several hoppers as Flue Gas goes to stack; Heavier particles fall out first due to turns in gas stream

Relative distribution of ash to various hoppers is not accurately known

Preferred way to collect a) a representative sample b) sample of the test period is to use High Volume Sampler probes on both sides of boiler

High Volume Sampler

This sampler uses 2-3 ksc air through an aspirator to create vacuum to pull out a large volume of flue gas & ash into probes canister; A filter catches the ash but allows the gas to pass through.

Bottom Ash Sampling

Bottom ash samples are collected every 15 minutes from the scrappers system during the test

In case of impounded hoppers, incremental samples are collected from bottom ash hoppers disposal line at slurry discharge end

Unburnt carbon is determined as LOI (Loss on Ignition)

FG

Economizer

FG

APHSamplingLocations

APH

ExpansionBellow

Test Locations - AH Inlet & Outlet

Inlet Sampling plane to be as close to AH as possible; Outlet grid to be a little away to reduce stratification

AH hopper / Manhole air ingress can influence test data

Sampling Ports in Flue Gas Ducts (Typical )

Sampling Point for Flue Gas Temperature & Composition

100mm

Gas Duct is divided into equal cross-sectional areas and gas samples are drawn from each center using multi point

probes or point by point traverse

HVT - High Velocity Thermocouple Probe - A Diagnostic Tool

To establish furnace gas exit temp profile

To establish CO & O2 profile at furnace outlet

To confirm proper distribution of fuel and air

To quantify air ingress between furnace outlet and AH inlet

Typical problems High Economiser / AH exit gas temperature Air ingress from furnace bottom, penthouse and

second pass Boiler operation at high excess air Metal temperature excursions High Unburnt carbon in ashes Uneven Flyash Erosion Flame failures Shortfall in steam temperatures Imbalance in Left - Right steam temperatures

Boiler Parametric Optimisation A structured exercise to evolve an optimum operating

regime for a boiler; a set of operating parameters and equipment settings for safe, reliable and efficient operation.

To establish interrelationships between different operating parameters.

To build a repeatable database for problem solving and diagnosis by various parametric tests.

All the more necessary when firing blended coals.

THANKS